Inkjet printing mechanisms use cartridges, often called "pens," which eject drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, ejecting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a "service station" mechanism is supported by the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, these service stations usually include a capping system which substantially seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting," with the waste ink being collected in a "spittoon" reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead. The wiping action is usually achieved through relative motion of the printhead and wiper, for instance by moving the printhead across the wiper, by moving the wiper across the printhead, or by moving both the printhead and the wiper.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment-based inks have been developed. These pigment-based inks have a higher solid content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to form high quality images on readily available and economical plain paper.
Early inkjet printers used a single monochromatic pen, typically carrying black ink. Later generations of inkjet printing mechanisms used a black pen which was interchangeable with a tri-color pen, typically one carrying the colors of cyan, magenta and yellow within a single cartridge. The tri-color pen printed a "process" or "composite" black image, by depositing drops of cyan, magenta, and yellow inks all at the same location. Unfortunately, the composite black images usually had rough edges, and a non-black hue or cast, depending for instance, upon the type of paper used. The next generation of printers further enhanced the images by using either a dual pen system or a quad pen system. The dual pen printers had a black pen and a tri-color pen mounted in a single carriage to print crisp, clear black text while providing full color images.
The quad pen printing mechanisms had four separate pens that carried black ink, cyan ink, magenta ink, and yellow ink. Quad pen plotters typically carried four pens in four separate carriages, so each pen needed individual servicing. Quad pen desktop printers were designed to carry four cartridges in a single carriage, so all four cartridges could be serviced by a single service station. As the inkjet industry investigates new printhead designs, there is a trend toward using permanent or semi-permanent printheads in what is known in the industry as an "off-axis" printer. In an off-axis system, the printheads carry only a small ink supply across the printzone, with this supply being replenished through tubing that delivers ink from an "off-axis" stationary reservoir placed at a remote location, typically inside a desktop printer, although large format plotters and industrial implementations may store their ink supplies external to the printing mechanism. The smaller on-board ink supply makes these off-axis desktop printers quite suitable for quad pen designs.
These earlier dual and quad pen printers required an elaborate capping mechanism to hermetically seal each of the printheads during periods of inactivity. A variety of different mechanisms have been used to move the servicing implements into engagement with respective printheads. For example, a dual printhead servicing mechanism which moves the caps in a perpendicular direction toward the orifice plates of the printheads is shown in U.S. Pat. No. 5,155,497, assigned to the present assignee, Hewlett-Packard Company, of Palo Alto, Calif. Another dual printhead servicing mechanism uses the carriage to pull the caps laterally up a ramp and into contact with the printheads, as shown in U.S. Pat. No. 5,440,331, also assigned to the Hewlett-Packard Company. A rotary device for capping dual inkjet printheads is commercially available in several models of printers produced by the Hewlett-Packard Company of Palo Alto, Calif., including the DeskJet.RTM. 850C, 855C, 820C and 870C model printers. Examples of a quad pen capping system that uses a translation motion are seen in several other commercially available printers produced by the Hewlett-Packard Company, including the DeskJet.RTM. 1200 and 1600 models. Thus, a variety of different mechanisms and angles of approach may be used to physically move the caps into engagement with the printheads.
The caps in these earlier service station mechanisms typically included an elastomeric sealing lip supported by a movable platform or sled. Typically, provisions were made for venting the sealing cavity as the cap lips are brought into contact with the printhead. Without a venting feature, air could be forced into the printhead nozzles during capping, which could deprime the nozzles. A variety of capillary passageway venting schemes are known to those skilled in the art, such as those shown in U.S. Pat. Nos. 5,027,134; 5,216,449; and 5,517,220, all assigned to the present assignee, the Hewlett-Packard Company.
The earlier cap sleds were often produced using high temperature thermoplastic materials or thermoset plastic materials which allowed the elastomeric sealing lips to be onsert molded onto the sled. The elastomeric sealing lips were sometimes joined at their base to form a cup-like structure, whereas other cap lip designs projected upwardly from the sled, with the sled itself forming the bottom portion of the sealing cavity. Unfortunately, the systems which used a portion of the sled to define the sealing cavity often had leaks where the cap lips joined the sled. To seal these leaks at the lip/sled interface, higher capping forces were used to physically push the elastomeric lip into a tight seal with the sled. This solution was unfortunate because these higher capping forces may damage, unseat or misalign the printhead, or at the vary least require a more robust printhead design which is usually more costly.
Capping systems need to provide an adequate seal while accommodating a several different types of variations in the printhead. For example, today's printhead orifice plates often have a waviness or ripple to their surface contour because commercially available orifice plates unfortunately are not perfectly planar. Besides waviness, these orifice plates may also be slightly bowed in a convex, concave or compound (both convex and concave) configuration. The waviness property may generate a height variation of up to 0.05-0.08 millimeters (2-3 mils; 0.002-0.003 inches). These orifice plates may also have some inherent surface roughness over which the cap must seal. The typical way of coping with both the waviness problem and the surface roughness problem is through elastomer compliance, where a soft material is used for the cap lips. The soft cap lips compress and conform to seal over these irregularities in the orifice plate. For instance, one earlier suspended lip configuration having a single upwardly projecting ridge for a sealing lip is shown in U.S. Pat. No. 5,448,270, assigned to the Hewlett-Packard Company, the present assignee.
Another major surface irregularity over which some printhead caps must seal are two encapsulant beads which attach each end of the silicon nozzle plate to a portion of an electrical flex circuit which delivers firing signals to energize the printhead resistors. An energized resistor heats the ink until a droplet is ejected from the nozzle associated with the energized resistor. These encapsulant beads project beyond the outer surface of the nozzle plates. In the past, caps were designed to avoid sealing over the encapsulant bead regions, either by sealing between the beads or beyond them. One printer design, the DeskJet.RTM. 693C color inkjet printer sold by the Hewlett-Packard Company of Palo Alto, Calif., has a capping system that accommodates interchangeable black and photo-quality color pens, either of which is used in combination with a standard tri-color pen. This capping system used a multiple sealing lip system to seal across (perpendicular to) the encapsulant beads.
One other earlier capping system, is currently commercially available in the DeskJet.RTM. 850C, 855C, 820C and 870C model color inkjet printers, sold by the Hewlett-Packard Company of Palo Alto, Calif. The capping system in these earlier printers used a multiple sealing lip system to seal along the length of the encapsulant beads. That is, in this earlier design the multiple sealing lips ran parallel to the encapsulant beads to accommodate for manufacturing tolerance accumulation and/or cap placement tolerance, so at least one of the multiple lips would land in a suitable location on the orifice plate to form a seal. Unfortunately, these fine multiple lips are very difficult to manufacture, Often the lips break off as they are removed from the mold, so the scrap rate is relatively high, which translates to a higher overall piece price for the printer manufacture. Indeed, only a few companies are even capable of consistently producing quality caps of this multi-lip design.
Proper capping requires providing an adequate hermetic seal without applying excessive force which may damage the delicate printheads or unseat the pens from their locating datums in the carriage. Moreover, it would be desirable to provide such a capping system which is more economical to manufacture than earlier capping systems, and which can be manufactured by a variety of vendors.